In an internal combustion engine (engine) used for automobiles and the like, a compressed self-ignition type gasoline engine (hereinafter, a compressed self-ignition engine), which compresses an air-fuel mixture to cause self-ignition combustion (compressed self-ignition type combustion), has gained attention as a means of concurrently achieving improvements in fuel economy and in emission performance. Compared with spark-ignition type combustion in which the air-fuel mixture is caused to ignite and burn with a spark of an ignition plug, a compressed self-ignition engine can concurrently achieve fuel economy and emission performance, since it reduces fuel consumption through an increased efficiency by high compression ratio, a reduction of pumping loss, and a reduction of cooling loss through fast combustion, etc. and also reduces NOx concentration in the exhaust gas through low temperature combustion of the air-fuel mixture.
Among the means of realizing compressed self-ignition type combustion is the introduction of EGR. While the spark-ignition type combustion is performable in a region where the air-fuel ratio is relatively rich and the EGR rate is relatively low, the compressed self-ignition type combustion when the above described means is adopted is performable in a region where the air-fuel ratio is relatively lean and the internal EGR rate is relatively high. Moreover, an unstable combustion region in which both types of combustion become unstable exists between the respective regions. It is noted that as the method for introducing EGR, a method of causing exhaust gas to remain in the cylinder by providing a negative valve overlap period which is the period in which both the intake valve and the exhaust valve are closed in an exhaust stroke (internal EGR), a method of admitting exhaust gas with fresh air by providing a bypass from the exhaust pipe in the upstream of the intake valve (external EGR), or a method of readmitting exhaust gas by opening the exhaust valve in an intake stroke (exhaust gas readmission) is used.
When the method of compressed self-ignition type combustion as described above is applied, since a large amount of exhaust gas is introduced into the cylinder, and thereby the fresh air quantity is limited, the engine torque that can be produced will be limited to lower loading if supercharging is not performed. Further, it is known that in each stroke of intake, compression, expansion, and exhaust, since it is necessary to ensure a finite time for the fuel to undergo chemical reaction, the engine rotational speed is also limited to a lower rotational speed.
Because of that, it is proposed that when a compressive self-ignition engine is applied to an automobile, both of spark-ignition type combustion and compressed self-ignition type combustion are performed so that those combustion types are switched to realize an engine torque that the driver demands.
In the compressed self-ignition type combustion, combustion is possible in an atmosphere in which the air-fuel ratio, which is the mass ratio between the air quantity and fuel quantity in the cylinder, is leaner (less fuel quantity) compared with the case of spark-ignition type combustion. Therefore, aiming at the reduction of fuel consumption and discharge quantity of NOx, the air-fuel ratio is set to be lean in the compressed self-ignition type combustion. This is realized by making the throttle opening full open to take in a large amount of air into the cylinder. But, since the fresh air quantity is limited in a naturally aspirated engine as described above, the air-fuel ratio in a compressed self-ignition type combustion region changes to the rich side (more fuel) as the engine torque increases. Particularly, when the air-fuel ratio becomes excessively rich, the progress of the oxidation reaction of fuel becomes difficult due to a decline of the concentration of oxygen and the temperature of the air-fuel mixture in the cylinder, and there may be case where the combustion stability declines eventually leading to misfire. From what is described above, it is often the case that the air-fuel ratio of the higher load side in the compressed self-ignition type combustion region is set at a near stoichiometric value.
While, in the compressed self-ignition type combustion, the throttle is fully opened to introduce a large amount of air into the cylinder to make the air-fuel ratio lean as described above, the air quantity that can be introduced varies depending on various conditions. Examples of such condition include surrounding environments such as the atmospheric pressure and air temperature, operating states such as manufacturing variation and deterioration of the intake and exhaust valves which perform gas exchange of the cylinder, and operating states of the injector which injects fuel.
If the air-fuel ratio becomes richer due to the change of the various conditions when trying to produce a higher engine torque by compressed self-ignition type combustion, the air-fuel ratio will become excessively richer than an initially planned air-fuel ratio and, as a result of that, the continuation of the compressed self-ignition type combustion becomes impossible, thus leading to a misfire. Moreover, if the air-fuel ratio becomes leaner, since the combustion can be performed at an air-fuel ratio which is leaner than initially planned, the compressed self-ignition type combustion region can be expanded toward the higher engine torque side, enabling further reduction of fuel consumption. However, if the compressed self-ignition type combustion region in the initial setting is maintained, its potential for reducing fuel consumption cannot be effectively utilized.
Thus, there is a problem in that to ensure the operability and reduce the fuel consumption with satisfaction in an engine that switches two combustion modes like a compressive self-ignition engine, the compressed self-ignition type combustion region must be changed according to the air-fuel ratio during compressed self-ignition type combustion.
As a method for solving this problem, a technology to change the combustion region based on the air-fuel ratio is known (for example, Patent Literature 1). Patent Literature 1 relates to the technology for switching between combustion in the combustion region which is performed at a relatively low engine torque, and combustion in the second combustion region which is performed at a relatively high engine torque in a diesel engine. To be specific, the engine torque upper limit in the combustion region is changed toward the higher engine torque side when the air-fuel ratio is rich, based on the air-fuel ratio during the operation in the combustion region. Moreover, when the air-fuel ratio is lean, the engine torque upper limit in the combustion region is changed toward the lower engine torque side.